The purpose of this study is (a) to isolate tumor-specific and virus-related antigens from a crude perchloric acid extract of MTV-induced mammary tumors of BALB/cfC3H mice, (b) to assess the biological activities of the isolated components, (c) to determine the cell types involved in the DNA synthetic response of tumor bearing mouse spleen cells to components of the extract, (d) to determine the changes of T cell, B cell and macrophage population with growth of syngeneic transplants of the tumor, (e) to determine the effect of tumor growth on DNA synthetic activity, (f) to determine the nature of serum factors inhibiting the DNA synthetic response and to isolate the serum inhibitory factors, (g) to determine the mechanism of action of the serum inhibitory factors, and (h) to determine the effect of surgical removal of the tumor on lymphocyte response to the tumor extracts, on the changes in T cell, B cell and macrophage populations, and on serum inhibition of DNA synthesis.